The present invention relates to a cam disk for a toroidal type continuously variable transmission used as a transmission of a vehicle such as an automobile.
Conventionally, a stage transmission which comprises speed change gears is used as a transmission of an automobile. This type of transmission has a plurality of gears. The combination of gears is changed to transmit torque from an input shaft to an output shaft at a desired transmission ratio. In the conventional transmission, torque is changed stage by stage, when the speed is changed. Thus, the conventional transmission is disadvantageous in that the power transmission efficiency is low and that vibration occurs when the speed is changed. To overcome these disadvantages, in recent years, a continuously variable transmission is put to practical use. With the continuously variable transmission, no vibration occurs when the speed is changed. In addition, since the power transmission efficiency is higher than that of the aforementioned conventional transmission, the fuel efficiency of the engine is improved.
As an example of the continuously variable transmissions, conventionally, a toroidal type continuously variable transmission 120 as shown in FIG. 14 has been proposed. This type of transmission comprises an input disk 102, an output disk 103a, power rollers 103b rotationally in contact with the disks 102 and 103a, a loading cam mechanism 106, etc. The input disk 102 is rotated in association with the input shaft 101. The input shaft 101 is connected to a drive shaft 122 rotated by an engine serving as a power source. The output disk 103a is rotated in association with an output shaft (not shown). The loading cam mechanism 106 presses the input disk 102 and the output disk 103a in such directions that the disks get closer to each other.
A toroidal type continuously variable transmission with a single cavity comprises a pair of an input disk 102 and an output disk 103a. A toroidal type continuously variable transmission with double cavities comprises two pairs of input disks 102 and output disks 103a. FIG. 14 shows a part of a double-cavity toroidal type continuously variable transmission 120. The transmission 120 has a first cavity 108 including first input and output disks 102 and 103a and power rollers 103b, and a second cavity including second input and output disks and power rollers (not shown). The loading cam mechanism 106 is provided, for example, on the side of a power source for driving the input disk 102 of the first cavity 108. The loading cam mechanism 106 has a cam disk 104 and a roller 105 serving as pressing means. The cam disk 104 is rotatably supported by an input shaft 101 via a ball 125. The roller 105 is rotatable between the cam disk 104 and the input disk 102 about an axis M1 crossing an axis P1 of the input shaft 101. The input disk 102 is pressed against the output disk 103a via the roller 105.
The cam disk 104 shown in FIG. 14 integrally comprises a first projecting portion 112, a second projecting portion 113, a flange portion 114 and a cam surface 115. The first and second projecting portions 112 and 113 are projected from a central portion of the disk 104 in both axial directions of the disk 104. The thickness of the flange portion 114 is gradually reduced from the first projecting portion 112 toward the peripheral portion. The roller 105 is brought into contact with the cam surface 115. In the central portion of the cam disk 104, a fitting hole 116 is formed, through which the input shaft 101 is inserted. A continuous raceway 117 is formed in the overall inner circumference of the fitting hole 116. A continuous raceway 118 is formed in the overall outer circumference of the input shaft 101. The raceways 117 and 118 have arc-shaped cross sections corresponding to the outer diameter of the ball 125.
A line segment N1 connecting bottoms 117a and 118b of the raceways 117 and 118 is inclined with respect to the axis P1 of the input shaft 101. When the first input disk 102 is pressed by the roller 105 in the direction toward the first output disk 103a, the counterforce is applied to the input shaft 101 via the ball 125, thereby pressing the input shaft 101 toward the power source. As a result, the second input disk (not shown) is pressed toward the second output disk. The input shaft 101 and the cam disk 104 are rotatable with respect to each other via the ball 125 rotatably held between the raceways 117 and 118.
The cam disk 104 comprises teeth 112a formed integral with an end portion of the first projecting portion 112. The teeth 112a mesh with teeth 122a formed in the drive shaft 122, so that the cam disk 104 is rotated together with the drive shaft 122. In other words, the rotation of the drive shaft 122 is transmitted to the cam disk 104 via the teeth 112a and 122a. As a result, the first input disk 102 and the second input disk are rotated. The rotation of the first input disk 102 is transmitted to the first output disk 103a via the first power roller 103b. The rotation of the second input disk is transmitted to the second output disk via the second power roller. As a result, the output axis is rotated.
The toroidal type continuously variable transmission 120 can transmit higher torque than the conventional belt type continuously variable transmission described above. However, considerable compressive stress and tensile stress act on the cam disk 104. More specifically, when the input disk 102 is pressed toward the output disk 103a by the roller 105, much greater compressive stress and tensile stress act on the cam disk 104 as compared to the case of a general mechanical member on which stress is exerted repeatedly, such as, a gear or a bearing.
Particularly in regions enclosed by the dot-chain lines H1 in FIG. 14, considerable compressive stress acts on the cam surface 115 and the raceway 117. Further, the outer circumference of the flange portion 114 of the cam disk 104 is warped away from the input disk 102 by the counterforce applied to the cam disk 104 when the roller 105 press the input disk 105 toward the output disk 103a. For this reason, great tensile stress acts on a region enclosed by the dot-chain line H2 in FIG. 14, i.e., a corner section 119 where the second projecting portion 113 intersects the cam surface 115. In the teeth 112a which mesh with the teeth 122a of the drive shaft 122, great compressive stress acts on a top end portion of the teeth 112a enclosed by the dot-chain line H3 in FIG. 14. Great tensile stress also acts on a root portion of the teeth 112a enclosed by the dot-chain line H4.
Conventionally, in one method for producing the cam disk 104 described above, a solid material 126 as shown in FIG. 15 or a hollow material is cut-worked. The material 126 is shaped into a column by, for example, rolling. In another method, the material is shaped into a form approximate to the cam disk 104 by forging, and subjected to the finishing process, such as grinding. In the method of producing the cam disk 104 by a cutting process from the material, the production yield is very low and a considerable period of time is required for the process. As a result, the production cost is increased.
The material 126, shaped through the steps of melting, casting and rolling, may contain a relatively much impurities in a portion 126a, 30% or less of the diameter of the material from the center. Further, the material 126, which has been subjected to plastic working such as rolling, has metal flows G formed along the axis I of the material 126. A metal flow means a line of texture formed in the metal when crystal grains are aligned in a direction during the process of plastically working the metal texture. The metal flow is also called a flow line. The texture obtained by a preferred orientation of the crystal grains is called deformation texture or fiber texture. Such texture has anisotropy and different mechanical properties depending on directions.
When the material 126 having the metal flows G as shown in FIG. 15 is cut-worked, thereby producing the cam disk 104 as shown in FIG. 16, metal flows G1 are formed along an axis I1 of the cam disk 104. In this case, the metal flows G1 are interrupted by the cam surface 115, the surface of the corner section 119, the surface of the raceway 117, etc., and so-called end flows E are formed. The angle .theta.10 between the cam surface 115 and the metal flows G1 is as large as, for example, 90.degree.. The angle .theta.11 between the tangent of the raceway 117 and the metal flow G1 is as large as, for example, 30.degree. or larger. Moreover, the central portion 126a of the material 126, containing a relatively great deal of impurities, may be exposed on the surface of the raceway 117.
Thus, in the cam disk 104 produced mainly by the cut-work process, a great deal of impurities may be contained or the end flows E may exist in the cam surface 115 and the surfaces of the raceway 117 and the corner section 119, on which much stress is exerted. In this case, the cam disk 104 is liable to break along the metal flows G1. This results in reduction in lifetime of the cam disk 104 and the toroidal type continuously variable transmission having the cam disk 104.
On the other hand, the material (work) 126 may be first shaped into a form approximate to the cam disk 104 by die forging and the n subjected to a cutting process. In this method, since only one kind of die is used, the metal flows cannot be formed along the cam surface 115 or the surfaces of the corner section 119 and the raceway 117. As a result, end flows a re formed on these surfaces. Moreover, the central portion 126a of the material 126, containing a relatively great deal of impurities, may be exposed on the cam surface 115 and the surfaces of the corner section 119 and the raceway 117. Therefore, the cam disk 104 formed by this method also tends to have a short lifetime. In addition, according to this method, the die used in forging is in contact with the work for a long period of time. For this reason, since the die is influenced by high heat generated during the forging process, the surface hardness of the die is lowered and the lifetime of the die is liable to be shortened. Further, in the case where the work (material 126) is shaped into a form approximate to the cam disk 104 by die forging, the conventional die does not have a structure for holding the work. Therefore, the work is easily displaced from the center of the die, with the result that the work accuracy may be lowered.
When die forging is performed with one kind of die, underfill, burr or flash is liable to occur in a corner inside the die. Therefore, it is difficult to shape the material to a desired form. To shape the material 126 to a form approximate to the cam disk 104 in one forging process, a high pressure is required. However, if an excessive pressure is applied, the die may be damaged. Further, to reduce the margin for cutting the work in a cutting process after the forging process, it is necessary to reduce wear of the die. Thus, in the method where first the material is shaped into a form approximate to the cam disk 104 by one die forging process and then subjected to a cutting process, the lifetime of the die may be reduced and the production cost may be increased.
In the double-cavity half toroidal type continuously variable transmission 120 described above, if the transmission torque of the first cavity 108 and the transmission torque of the second cavity are different, simultaneity in changing the speed of the two cavities may be adversely influenced. In addition, if the transmission torques of the cavities are different, one of the cavities must transmit torque greater than the design value. In this case, slippage occurs on contact surfaces between the power roller 103b and the disks 102 and 103a. For these reasons, it is desirable that the transmission torques of the first cavity 108 and the second cavity be equal so far as possible.
The cam disk 104 produced by the conventional method as described above has end flows on the surface of the raceway 117. Therefore, when the ball 125 is rotated while it is in contact with the surface of the raceway 117 at a high pressure, flaking easily occurs on the surface of the raceway 117. When flaking occurs, the friction between the ball 125 and the raceway 117 is increased. When the friction between the ball 125 and the raceway 117 is increased, the first input disk 102 is liable to rotate along with the cam disk 104. As a result, the transmission torques of the first cavity 108 and the second cavity become different, which is not preferable for the reason described above.
Further, in the teeth 112a, as described above, considerable compressive stress acts at the distal end portion and considerable tensile stress acts on the root portion. Therefore, the teeth 112a tend to break along the metal flows, resulting in reduction in lifetime of the cam disk 104.